linux-kernel - Re: [PATCH 1/8] perf: Allow to block process in syscall tracepoints

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Date:   Thu, 6 Dec 2018 13:19:46 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Jiri Olsa <jolsa@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Luis Claudio R. Goncalves" <lclaudio@...g.org>, ldv@...linux.org,
        esyr@...hat.com, Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH 1/8] perf: Allow to block process in syscall tracepoints

On Thu, 6 Dec 2018 09:34:00 +0100
Peter Zijlstra <peterz@...radead.org> wrote:

> > 
> > I don't understand this.. why are we using schedule_timeout() and all
> > that?  
> 
> Urgh.. in fact, the more I look at this the more I hate it.
> 
> We want to block in __perf_output_begin(), but we cannot because both
> tracepoints and perf will have preemptability disabled down there.
> 
> So what we do is fail the event, fake the lost count and go all the way
> up that callstack, detect the failure and then poll-wait and retry.
> 
> And only do this for a few special events...  *yuck*

Since this is a special case, we should add a new option to the perf
system call that, 1 states that it wants the traced process to block
(and must have PTRACE permission to do so) and 2, after it reads from
the buffer, it needs to check a bit that says "this process is blocked,
please wake it up" and then do another perf call to kick the process to
continue.

I really dislike the polling too. But because this is not a default
case, and is a new feature, we can add more infrastructure to make it
work properly, instead of trying to hack the current method into
something that does something poorly.

-- Steve